1. Field of the Invention
The present invention generally relates to a surface acoustic wave resonator, a ladder-type surface acoustic wave filter having surface acoustic wave resonators arranged in a series arm and a parallel arm, and a composite type surface acoustic wave filter in which a surface acoustic resonator is combined with a double mode type surface acoustic filter or the like.
2. Description of the Related Art
A ladder-type surface acoustic wave (SAW) filter having a plurality of SAW resonators is known as a band-pass filter (see Transaction A of the Institute of Electronics, Information and Communication Engineers, Vol. J 76-A, No.2, 1993, pp. 245-252).
FIG. 1 shows a conventional ladder-type SAW filter. The filter includes a piezoelectric substrate 10 on which SAW resonators S1 and S2 are respectively arranged in series arms between an input terminal Ti and an output terminal To and SAW resonators P1 and P2 are respectively arranged in parallel arms. One of the parallel arms is provided between the input terminal Ti and ground G, and the other parallel arm is provided between the node between the SAW resonators S1 and S2 are connected and ground G. Each of the SAW resonators S1, S2, P1 and P2 is called one-port SAW resonator.
FIG. 2 illustrates a structure of the one-port SAW resonator. The one-port SAW resonator includes an interdigital transducer (hereinafter simply referred to as IDT) 11 and two reflectors 12 and 13, all of which are formed on the piezoelectric substrate 10. The IDT 11 electrically excites a SAW. The reflectors 12 and 13 are positioned on SAW propagation paths and act as confine the SAWs excited by the IDT 11. In case where a desired resonance characteristic is obtained by utilizing internal reflection of SAW by the IDT 11 alone, the reflectors 12 and 13 may be omitted.
The IDT 11 has a pair of comb electrodes, each of which has a number of electrode fingers arranged with a fixed period Pi. Each of the reflectors 12 and 13 has a number of grating electrodes arranged with a fixed period pr, and is called grating reflector.
A pair of adjacent electrode fingers respectively extending upwards and downwards forms a unit for excitation of SAW. An electrode arrangement having two electrode fingers within one period pi is called single electrode or single electrode arrangement.
When one electrode finger of the single electrode arrangement has a width w, this electrode finger has a pattern width of 2w/pi×100(%). When the electrode finger width w is equal to a space width s between the adjacent electrode fingers (w=s) as shown in FIG. 2, the pattern width is 50%. An average pattern width means the average of the pattern widths of all the electrode fingers that form the IDT.
Generally, the electrode fingers that form the IDT 11 of the one-port SAW resonator are designed to have a pattern width of 50% for the purpose of reducing the resistance of the electrode fingers and the amount of frequency shift dependent on variation in the pattern width introduced during the production process.
FIG. 3A shows a frequency characteristic of the conventional SAW resonator mentioned above. Generally, the SAW resonator has properties of dual resonance that has a resonance frequency fr and an anti-resonance frequency fa. An arrangement having SAW resonators connected in series acts as a low-pass filter, as shown by a solid line in FIG. 3A. In series connection, insertion loss is minimized at a resonance frequency frs and is maximized at an anti-resonance frequency fas. An arrangement having SAW resonators connected in parallel acts as a high-pass filter, as shown by a broken line in FIG. 3A. In parallel connection, the insertion loss is maximized at a resonance frequency frp and is minimized at an anti-resonance frequency fap.
The ladder-type SAW filter is made up of SAW resonators S1 and S2 arranged in series arms and SAW resonators P1 and P2 arranged in parallel arms, as shown in FIG. 1. Therefore, a band-pass filter having a pass band as shown in FIG. 3B is available by designing the IDTs of the SAW resonators so that the anti-resonance frequencies fap of the SAW resonators P1 and P2 are approximately equal to the resonance frequencies frs of the SAW resonators S1 and S2.
FIG. 4 shows required band characteristics of the band-pass filter such as the ladder-type SAW filter. The characteristics can be described by desired bandwidths (BW1, BW2), the degrees of suppression (ATT1, ATT2) at frequencies defined in the specification, and the widths of the suppressed ranges (BWatt1, BWatt2). Further, the characteristics can be evaluated by a shape factor that describes the ratio of the bandwidth BW1 at a given attenuation to the bandwidth BW2 at another given attenuation (BW1/BW2). The filter characteristics become better as the shape factor becomes closer to 1.
The shape factor of the ladder-type SAW filter may be improved by sharpening the transition slopes that connect the attenuation ranges and the pass-band range. The sharpness of the transition slopes is almost determined by a difference Δf between the resonance frequency fr and the anti-resonance frequency fa of the SAW filter. Decreasing the frequency difference Δf sharpens the transition slopes. Various methods for decreasing the frequency difference Δf have been proposed (for example, Japanese Unexamined Patent Publication No. 11-163664). As shown in FIG. 5, the falling slope can be sharpened by reducing the difference Δfs between the resonance point and anti-resonance point of the SAW resonator in the series arm, so that the shape factor can be improved.
However, the methods for reducing the frequency difference Δf to thereby sharpen the transition slopes reduce the pass-band width of SAW passage. Therefore, there is a tradeoff relationship between improvement in the shape factor and expansion of the pass-band width and there is difficulty in achieving concurrent improvements.